An eyepiece is an optical system which, when used in conjunction with another optical system producing a real image (the objective), produces a real or virtual image at large distance from the eyepiece suitable for viewing by the eye, or for introduction into another optical system focused at large distance. The input pupil of the objective should be imaged into a location outside the eyepiece acessible to the eye or other system (e.g. diaphragm of a camera lens).
Eyepieces are subject to the same aberrations as other optical systems. These include the third order Seidel aberrations: spherical aberration, coma, astismatism, curvature of field, and distortion. In addition eyepieces exhibit longitudal and lateral chromatic aberrations due to the variation of the refractive index of glass with wavelength. Another aberration, unique to eyepieces, is spherical aberration of the pupil wherein the position of the image of the objective input pupil varies with field angle. The invention disclosed here succeeds in correcting the first four Seidel aberrations (hence it is an anastigmat) along with the chromatic aberrations and exhibits a very favorable pupil location.